1. Field of the Invention
The present invention relates to determination of continuous capillary pressures for rock in subsurface formations from well logs.
2. Description of the Related Art
Capillary pressure is an important input to reservoir simulators. Unfortunately, its determination using current traditional methodologies, namely mercury injection capillary pressure experiments, has been relatively expensive, slow, and not environmentally friendly. Because of that and the fact that physical rocks from the subsurface were required, only sparse data points have usually been analyzed. Engineers were then forced to populate their models with a handful of curves or use curves from analogous reservoirs. This, in turn, could lead to gross errors in the simulation results if a proper petrophysical classification of rocks had not been conducted.
The use of nuclear magnetic resonance (NMR) to obtain capillary pressure curves have been investigated in the past. Results were encouraging if a laboratory measured NMR response is used. However, the use of a NMR wireline response has been questionable due to numerous complexities, such as the existence of a hydrocarbon effect. In addition, calibration of NMR derived capillary pressure curves usually required physical rock samples which was sometimes undesirable for cost reasons. Current NMR based methods have also, so far as is known, generally required calibration using methods such as mercury injection capillary injection (MICP) experiments which were constraining in terms of time, money and the number of samples.